Filmes finos de óxido de tungstênio obtidos por anodização: estrutura e propriedades fotoluminescentes e fotocatalíticas

Abstract

Tungsten oxide thin films were obtained by anodization, under constant voltage. Firstly, the reduction of the NaF concentration was investigated; the hypothesis that NaCl in aqueous solution, which under proper voltage produces gases H2 and Cl2 and alkaline solution, would lead to the formation of porous oxide, inspired the investigation of the NaF replacement by NaCl. Sodium fluoride, even in lower concentrations than that presented in literature, produced nanostructured films, with high photoluminescence (PL) intensity from 300 to 640 nm; sodium chloride, even in low concentration, promoted the formation of films with excellent photocatalytic efficiency for methyl orange dye degradation, increasing the rate constant up to 168% in relation to the uncatalysed system. In the second step, the anodization conditions in NaCl were optimized by a factorial design 24 matrix, with variables: NaCl concentration, presence of glycerol, applied tension and thermal treatment (300°C, 60 min). The main parameter that influences the PL intensity, evaluated at 500 nm, is the electrolyte concentration and low concentration is recommended when the maximization of emission yielding is desired. Heat treatment exerts the greatest influence on the particle size, producing larger structures, with textures walnut type, predominantly crystalline. The films did not show satisfactory photocatalytic performance for methylene blue dye degradation. The modifying agents Na2HPO4 and glycerol do not present significant effects on structuring of these materials. Films not thermally treated presented granular structure with diameters between 46 and 120 nm, higher amorphous content and, therefore, present lower refractive indexes than those heat-treated.